1. Field of the Invention
This invention relates to an exhaust gas recirculation control system for internal combustion engines, and more particularly to an exhaust gas recirculation control system which recirculates part of exhaust gases emitted from the engine to the intake system thereof, to thereby decrease emission of noxious components in the exhaust gases, such as NOx.
2. Prior Art
Conventionally, there has been proposed a method of controlling exhaust gas recirculation, for example, by Japanese Provisional Patent Publication (Kokai) No. 60-173361, which method comprises providing an exhaust gas recirculation passage for recirculating part of exhaust gases emitted from an internal combustion engine to the intake system thereof, and an exhaust gas recirculation valve for controlling the flow rate of exhaust gases to be recirculated through the exhaust gas recirculation passage, and calculating a desired amount of exhaust gases to be recirculated in response to the air-fuel ratio of a mixture supplied to the engine, to thereby control the exhaust gas recirculation valve, based on the calculated desired amount of exhaust gases to be recirculated.
The above conventional method, however, has the following inconvenience: For example, when the air-fuel ratio of the mixture has been changed from a rich state to a lean state, NOx is emitted in large amounts. To reduce the emission of NOx, a large amount of exhaust gases should be supplied by recirculation to the engine. However, the method does not contemplate a so-called "dynamic delay" of exhaust gases to be recirculated, which is a time lag from the time the amount of exhaust gases to be recirculated is calculated to the time the calculated amount of exhaust gases is actually supplied to the engine. Therefore, the desired amount of exhaust gases to be recirculated is not supplied to the engine immediately after the above change in the air-fuel ratio, resulting in failure to reduce the amount of NOx to a satisfactory degree immediately after the change.